Preparation of polyvinyl alcohol



June 16, 1953 G. P. WAUGH EIAL 2,642,419

' PREPARATION OF. POLYVINYL ALCOHOL Filed Jan. 51, 1950 QR POLYV/NYLESTER- ALCOHOL ALcoHoL/c ALKALI MIX/N6 CHAMBER awllvotk HOLDING ANDMSH/NG' m/VK GEORGE P. WAUGH ML IAM 0. wow

INVENTORS Mi W BYAJW Patented June 16, 1953 PREPARATION OF POLYVINYLALCOHOL George P..Waugh and William 0. Kenyon, Roch ester, N. Y.,assignors to Eastman Kodak Company, Rochester, N. Y., a corporation ofNew Jersey Application January 31, 1950, Serial No. 141,450

This inventionrelates to polyvinyl alcohol, and more particularly toimprovements in production'and purification of polyvinyl alcohol. Morespecifically,.the invention relates to. an

improved method of preparingahighly purified and substantially fullyhydrolyzed polyvinyl alcohol which is suitable for use in the productionof photographic emulsions or for use in other instances which requiresuch refined polyvinyl alcohols.

In the Kenyon et al. U. S. patent application Serial No. 36,816 filedJuly 2, 1948, are disclosed and claimed basic procedures for thedeacetylation of polyvinyl esters and for removing impurities from theproduct to insure a polyvinyl alcohol of highest purity. The presentapplication is distinguished in that it utilizes some of the processesof the co-pending invention together with newly discovered processes forpreparing a uniform quality of polyvinyl alcohol on a largemanufacturing scale.

In the art of photography, it has been found that the quality of thesilver halide emulsion vehicle, usually gelatin, has a pronounced effectupon the sensitivity and'keeping properties of the finished product.When polyvinyl alcohol is used as the emulsion vehicle, similar efiects,

are observed and it is therefore most essential that a uniform grade ofpolyvinyl alcohol be available. Therefore, one object of our inventionis to provide a method for prepar'inga high grade of polyvinyl alcoholof uniform quality. Another object is to provide a polyvinyl alcoholhaving low residual acetyl content. A further object is to provide apolyvinyl alcohol gel having minimum solubility in cold water. Anotherobject is to provide polyvinyl alcohols uniformly soluble at moderateand elevated temperatures. Other objects are set forth in the co-pendingapplication and further objects will become.

apparent from the following description of our invention. These objectsare in general accomplished by the deacylation of polyvinyl esters"bymeans of alkaline alcoholysis as opposed to 7 4 Claims (Cl. 260-913)agrammatic view of the methods andapparatus used in our invention.

According to-the broadest concept of our in-' vention we initiate thedeacylation of a synthetic resin having acyl groups in a solvent for theresin which is not a solvent fOr the deacylated resin,

by introducing a deacylation catalyst, preferably alkali metalhydroxide, into the solvent solution of the resin, then while. thesolution is in the fluid state depositing it in or upon the surface of acarrier Where it is allowed to gel, and then removing the gel from thecarrier and comminuting it-and further purifying it.- Toour knowledge,it has not previously been described to initiate such a reaction andthen to utiliz the resultant purely chemical change in a reactantthereof to produce a particular change in physical form.

In accordance with a more specific concept of theinvention, a polyvinylalcohol in gel form which is resistant to dissolving or 'sliming inWater can be prepared by vigorously mixing alcoholic solutions-ofpolyvinyl ester and alkali in a mixing chamber of the design similar tothat shown in the drawings. Thorough mixing is an essential featur ofour process; otherwise as a whole maybe unduly high in order to dis--solve the least readily soluble portions and conversely those portionsless completely deacetylated may be soluble in water and lost duringwashing. The alcoholusecl is preferably'methyl' alcohol although otheralcohols such as ethyl alcohol can be used and the alkali is preferablysodium hydroxide,-although other alkali metal hydroxides as well asalkali metal alcoholates are useful, as disclosed in the parentapplication. Similarly, the water content of the alcohol should not bemore than about 10 to 12% and preferably not over 5%.

The mixing procedure is also critical in the respect that the mixingtime should be so correlated With the balance of, the process thatsufficient time will remain before the composition sets to a gel thatthe composition may be deposited in the fluid state on to the carriersurface. be carriedout for from about ;01 to 0.1 of the total timerequired for the compositionto set to a gel for a mixer of the typeshown in the drawings. Other types of mixers with less holdup wouldrequire correspondingly less mixing Generally, the mixing operationcantime. lwixing appreciably longer may result in undue plugging of themixer and feed line from the mixing chamber. The quality of the productmay be adversely affected if the gel remains on the carrier overlong andbecomes too firm, an undesirable decrease in solubility in water takingplace in the subsequent grinding step due to milling action. Similarly,the concentration of alkaline catalysts and the temperature of themixture on the carrier surface should be carefully regulated to obtainthe optimum structure of gel for grinding. We prefer to use atemperature and catalyst concentration such that the gelling time isapproximately 3 to 6 minutes. A gelling time substantially less than 3minutes, e. g., 2 minutes, can be used. However, a gel is obtained whichis not as strong or as waterresistant, presumably because the moleculesare not properly oriented within the gel structure. Consequentlyappreciable amounts of gel may be lost in the washing step. Longergelling times can be used within the limitations of the equipment suchas the speed and length of the conveyor.

After completion of the mixing operation, the reaction mixture isdeposited on a surface, preferably the surface of a moving carrier, in asuit able manner, for example, by pumping from the mixing chamber andcoating on the carrier or as shown in the drawings by gravity fiow fromthe mixing chamber to the carrier. As shown in the drawings, thepreferred carrier is a continuous recessed moving belt; that is, thebelt is supported by a trough flared at both ends and the belt conformsto the contour of the trough. The result is that the mixture deposits insubstantial thickness on the belt and as it is carried away from theoutlet of the mixing chamber, it gels in the shape of the trough.Alternately, the belt can be provided with a series of recesses andprovision can be made for ejection of the gel after gelation. A suitablebelt is canvas coated with polyvinyl alcohol followed by a layer ofalcohol-resistant material such as cellulose ester or a resin,especially a vinyl chloride-vinylacetate copolymer.

The reaction mixture is carried by the belt at a suitable speeddepending upon the length of the carrier and the gelation time remainingafter mixing, A representative speed is 22 inches per minute under theconditions of Example 1 following. The mixture may be expected tocommence to show signs of gelation one-half to twothirds of the way tothe take-off point of the carrier and at the take-off point thecomposition has solidified to a gel and may be starting to show signs ofsyneresis. At this point before substantially any syneresis has takenplace, we remove the gel from the belt and at once pass it into thegrinder, cubing machine or other apparatus for subdividing the gel inthe required manner.

The point at which the gel is comminuted is an important feature of ourinvention/ We have found that if syneresis is allowed to proceed for anyappreciable time before grinding and the gel becomes appreciably harderthereby, the subsequent comminuting operation may have a mechanicalworking effect the result of which is to raise the minimum solutiontemperature of the gel in water. Consequently, for maximum quality weprefer to remove the gel from the carrier before substantial syneresishas occurred which is after a period of time has elapsed of about twotimes the gelling time of the catalyst-dope mixture. On the other hand,if the gel is ground too soon, when it is sticky, the particles willcohere in the grinder.

Following comminution we can allow the subdivided gel to synerese; thesupernatant liquid, including alcohol and methyl acetate, beingwithdrawn and the particles of gel are washed until free of salt. Ifdesired, for the purpose of further deacetylation as disclosed in theparent application, Example 21 thereof, before washing but aftersyneresis for at least one hour, with or without withdrawal of theliquid of syneresis, an additional quantity of alcoholic alkali may beadded to the comminuted gel.

At this point, a syneresis period of one hour or more is important sincethis increases the strength of the gel and its resistance to water. Afresh gel which has not undergone syneresis will be soluble in water atpractically any temperature. During the syneresis period the particlesof gel are allowed to stand if desired with gentle stirring, for about 1to 2 days to complete the deacetylation, then the liquid of syneresis isdrained off and the el washed with water. If desired, the addition offresh catalyst may be in the form of an aqueous alkaline solution asdisclosed in the parent application. Most of the methyl acetate shouldbe washed out before such treatment. The result of this procedure is toobtain a gel having lower residual acetyl content than obtained whenadding the catalyst in alcoholic solution. The minimum solutiontemperature is defined as the temperature at which a sample of gel whenstirred vigorously in water becomes a smooth solution.

Our invention will now be described by means of the following exampleswhich, together with the examples of our parent application and thedrawings, will serve to illustrate our invention:

Example I point and later on in the process is important since smallchanges in temperature have an appreciable efiect upon the rate ofalcoholysis. The

. solutions are essentially anhydrous although they can contain theindicated amount of water. The resin dope and the alkaline catalystsolution are metered into the mixing chamber by means of gear pumps inthe proportion of one volume of catalyst solution to ten ofdope. Asuitable pumping speed is 640 cc. of dope per minute. The mixing chamberis provided with an efficient propeller for mixing the ingredients andan outlet.

valve as indicated in the drawings to control the flow of the mixture tothe conveyor. As shown in the drawings, the mixer is located at one endof a trough in which the belt runs. The trough is 9.5 feet long and ismade of light gauge stain- 1 by flanges and is covered with a glass ormetal plate to minimize evaporation for the reasons set forthhereinafter. The belt is made of canvas,

with cold water for two days or more.

preferably coated with polyvinyl alcohol and then a layer of vinylchloride-vinyl acetate copolymer although other surfaces can be used aswell as other belts such as stainless steel. The belt runs over thepulleys as indicated and can be driven at various speeds. The mixture ofresin and catalyst setsto a gel about half-way down the trough and ifnecessary, this can be hastened by using a larger proportion of alkalior by operating-at a slightly higher temperature, preferably below theboiling point of the mixture. During the latter half of the trough thegel becomes reasonably firm and has hardened to the extent that it is nolonger sticky. This is particularly true if the mixture is allowed togel in a closed atmosphere designed to retard evaporation of solventfrom the surface. That is, we have found that if the dopecatalystmixture is allowed to gel on the conveyor with the exposed surface freeto the air, the mixture sets to a gel which is progressively softer fromthe conveyor surface outward and the latter Y surface may be slightlysticky at the take-01f point. This effect is presumably due to thecoolingeffect of evaporation of solvent from the surface of thecomposition. To compensate for this effect, We prefer to allow themixture'to set to a gel in a closed atmosphere retarding surfaceevaporation and in the present example this may be accomplished bycovering the trough with a metal, glass or other suitable plate as shownin o the drawings.

At the take-off point of the conveyor, as

syneresis is starting, the gel is removed from the through the bottom ofthe holding tank or in any other suitable manner and the gel is washedThe result is to obtain a polyvinyl alcohol gel substantially free fromacetyl and salts and having a minimum solution temperature ofapproximately 75 C.

Example 2 In a manner similar to that of the above example, a 24 percent solution of polyvinyl acetate in methanol was run into the mixingchamber at a rate of 52 cc. per minute, at the same time introducing a10 per cent solution of sodium hydroxide in methanol into the mixingchamber at approximately 6.1 cc. per minute. The mixture was then pumpedcontinuously by means of a gear pump from the mixing chamber through anextrusion apparatus from which it emerged as a gel whichwas comminutedand treated further as described in the above example.

While the belt conveyor method comprises a preferred embodiment of ourinvention, we are not limited thereto as a means for receiving andconveying the dope-catalyst mixture during the gelling period, since wecan instead use, for example, a circular moving trough provided with ascoop continuously removing the gel as the trough revolves, the freshdope-catalyst mixture being continuously deposited in the trough back ofthe scoop. Similarly, we can use a screw or similar conveyor, the gelemerging at the required rate from the outlet of the conveyor.

Similarly, a stationary surface or 'a' series of stationary receptaclescan be used for receiving the reaction mixture from a moving hopper orsimilar equipment. Conventional wheel coating methods may also beutilized. Other conveyor means will I occur to those skilled in the art.

In view of the fact that'the acid catalyzed" the viscosity of thepolyvinyl alcohol or for other reasons indicated in our parentapplication.

The advantages of our novel process are now apparent. Since we initiatethe alcoholysis of polyvinyl acetate and conduct the gelation andcomminuting steps in a continuous manner, we are not subject to thedifficulties of maintaining uniform quality of product which is typicalof batchwise operation and uniformity of product is thereby assured. Ourprocess is not simply the conversion of an old batch process to acontinuous process, especially since our process produces novelpolyvinyl alcohols; that is, of the type washable in water. Moreover,our process of laying out the reaction mixture on a conveyor where itassumes substantial thickness, entails a mechanical fabrication of areaction mixture which is a novel step.

Procedures other than those specifically set forth in the instantinvention and disclosed in the mentioned co-pending application may beintroduced into our process formodifying the character of the finalpolyvinyl alcohol; for example, the acetal content and color of thepolyvinyl alcohol maybe reduced. The alkaline catalyst, the alcohols,and polyvinyl esters useful in our process are those disclosed in theparent application.

Our invention having been described, we would have it understood thatthe disclosure herein is by way of example and that we consider asincluded in our invention all modifications and equivalents fallingwithin the scope of the appended claims.

We claim:

1. A method for the continuous manufacture of polyvinyl alcohol whichcomprises preparing an essentially anhydrous alcoholic solution of analkali metal hydroxide of the group consisting of lithium, sodium andpotassium hydroxides, and an essentially anhydrous alcoholic solution ofpolyvinyl acetate, simultaneously introducing said solutions into amixing chamber and thoroughly mixing said solutions, while the resultantmixture is in a fluid state continuously flowing it out onto an inertsurface continuously moving away from the source of mixing,- allowingthe mixture to form a gel. on said surface and after approximatelytwicethe gelling time of said mixture and before substantial syneresis hastaken place continuously removing the gel from the surface andimmediately thereafter comminuting the gel, allowing the comminuted gelto synerese for at least one hour and washing it with water at atemperature below the minimum solution temperature of the gel.

2. A method for the continuous manufacture of polyvinyl alcohol whichcomprises preparing an essentially anhydrous alcoholic solution of analkali metal hydroxide of the group consisting of lithium, sodium andpotassium hydroxides, and an essentially anhydrous alcoholic solution ofpolyvinyl acetate, simultaneouslyintroducing said solutions into amixing chamber and thoroughly mixing said solutions, while the resultantmixture is in a fluid state continuously flowing it out into an inertsurface continuously moving away from the source of mixing in anatmosphere retarding solvent evaporation, al-

anhydrous methanol solution of sodium hydroxide, and an essentiallyanhydrous methanol solution of polyvinyl acetate, simultaneouslyintroducing said solutions into a mixing chamber and thoroughly mixingsaid solutions, while the resultant mixture is in a fluid statecontinuously flowing it out onto an inert surface continuously movingaway from the source of mixing in an atmosphere retarding solventseparation, allowing the mixture to form a gel on said surface and afterapproximately twice the gelling time of said mixture and beforesubstantial syneresis has taken place continuously removing the gel fromthe surface and immediately thereafter comminuting the gel, allowing thecomminuted gel to synerese for at least one hour and washing it withwater at a temperature below the minimum solution temperature of thegel.

4. A method for the continuous preparation of polyvinyl alcohol whichcomprises preparing an essentially anhydrous methanol solution ofapproximately 10% sodium hydroxide and an -essentially anhydrousmethanol solution of approximately 24 to 30% polyvinyl acetate, simultaneously introducing said solutions into a mixing chamber in theproportion of approximately one volume ofsodium hydroxide solution toapproximately 10 volumes of polyvinyl acetate solution and thoroughlymixing the solutions, while the resultant mixture is in a fluid statecontinuously flowing it out onto an inert surface continuously movingaway from the source of mixing in an atmosphere retarding solventevaporation, allowing the mixture to form a gel on said surface andafter approximately twice the gelling time of said mixture and beforesubstantial syneresis has taken place continuously removing the gel fromthe surface and immediately thereafter comminuting the gel, allowing thecomminuted gel to synerese for at least one hour and washing it withWater at a temperature below the minimum solution temperature of thegel.

GEORGE P. VVAUGH. WILLIAM 0'. KENYON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName 7 Date 2,356,896 Smith Aug. 29, 1944 2,399,653 Roland May 7, 19462,467,774 Plambeck Apr. 14, 1949 2,499,097 Howk et al. Feb. 28, 1950OTHER REFERENCES Blout et al., Journ. Amer. Chem. 800., vol. 70, Feb.1948, pp. 862 to 865.

Karrer, Organic Chemistry, 2nd English E'dition, Elsevier, 1946, page106.

1. A METHOD FOR THE CONTINUOUS MANUFACTURE OF POLYVINYL ALCOHOL WHICHCOMPRISES PREPARING AN ESSENTIALLY ANHYDROUS ALCOHOLIC SOLUTION OF ANALKALI METAL HYDROXIDE OF THE GROUP CONSISTING OF LITHIUM, SODIUM ANDPOTASSIUM HYDROXIDES, AND AN ESSENTIALLY ANHYDROUS ALCOHOLIC SOLUTION OFPOLYVINYL ACETATE, SIMULTANEOUSLY INTRODUCING SAID SOLUTIONS INTO AMIXING CHAMBER AND THOROUGHLY MIXING SAID SOLUTIONS, WHILE THE RESULTANTMIXTURE IS IN A FLUID STATE CONTINUOUSLY FLOWING IT OUT ONTO AN INERTSURFACE CONTINUOUSLY MOVING AWAY FROM THE SOURCE OF MIXING, ALLOWING THEMIXTURE TO FORM A GEL ON SAID SURFACE AND AFTER APPROXIMATELY TWICE THEGELLING TIME OF SAID MIXTURE AND BEFORE SUBSTANTIAL SYNERESIS HAS TAKENPLACE CONTINUOUSLY REMOVING THE GEL FROM THE SURFACE AND IMMEDIATELYTHEREAFTER COMMINUTING THE GEL, ALLOWING THE COMMINUTED GEL TO SYNERESEFOR AT LEAST ONE HOUR AND WASHING IT WITH WATER AT A TEMPERATURE BELOWTHE MINIMUM SOLUTION TEMPERATURE OF THE GEL.